The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to a highly portable marine seismic vessel for accessing relatively inaccessible regions to deploy seismic streamers behind the vessel, and to collect geophysical data representing subsurface geologic formations.
Marine seismic vessels tow multiple seismic streamers through water to carry acoustic sensitive hydrophones. Acoustic energy sources such as air guns discharge energy pulses which travel downwardly into subsurface geologic formations underlying the water. Portions of the source energy are reflected upwardly by geologic structures and by the interfaces between adjacent formations. The acoustic signals detected by the hydrophones are converted into signals representing subsurface formation structures, and are recorded for data processing and display.
Marine seismic vessels require certain carrying capacity and space. Large arrays of multiple streamers up to several kilometers in length are towed behind seismic vessels to reduce the number of passes required by the vessel for the particular survey site. The streamers and combined streamer arrays are deployed and retrieved from the seismic vessel deck, requiring cable handling equipment and deck storage space. Work crews typically require sixteen members or more to handle multiple tasks, and the logistics of supporting crew members require vessel space.
The economic operation of seismic vessels depends on the number of days required for mobilization and demobilization. Because the nature of marine seismic exploration inherently covers large areas in remote regions, transport to the survey site significantly affects efficient utilization of a seismic vessel. Large seismic vessels capable of towing large streamer arrays are typically assigned to a particular geographic region having large water surfaces. However, large seismic vessels are not typically suited for Arctic regions having limited sailing seasons, or for regions having shallow water and multiple underwater obstructions. For seismic operations in the Beaufort Sea and other Arctic regions, water passage through the pack ice does not open every year. In heavy ice years, survey operations must be postponed until the next season or expensive icebreaking operations must be undertaken to provide passage. Even if a seismic vessel successfully passes through the ice flows to reach the survey site, the prospect of having the seismic vessel trapped by the next season's ice typically requires a conservative, abbreviated operating season. For Arctic seasons having a limited two or three month sailing season, the significance of each operating day is magnified.
If land masses and underwater obstructions prohibit operation of a large seismic vessel, shallow draft barges towed by a tug vessel can provide a floating base for conducting seismic operations. Such barges have limited deck space and do not provide crew quarters and other room essential to continuous operation of seismic operations. Accordingly, work crews commute between living quarters and the seismic barge, which exposes the crew to bad weather and other local hazards. In the Arctic and other extreme regions, fog, waves, floating ice, and other environmental hazards hinder crew travel.
Portable pontoon systems have been constructed to establish temporary bridges, docks, drilling platforms, and other floating bases to support equipment and other structural components. For example, U.S. Pat. No. 4,890,959 to Robishaw et al. (1990) disclosed a system for transporting ISO standard freight sized containers to a remote site and for assembling such containers into a structural base. U.S. Pat. No. 5,664,517 to Brydel et al. (1997) disclosed a pontoon connector system for permitting pontoon assembly under rough sea conditions.
Other systems provide assembled barge units designed for water transport. For example, U.S. Pat. No. 4,809,636 to Robishaw et al. (1989) and U.S. Pat. No. 4,928,616 to Robishaw et al. (1990) described a construction transportation system assembled with portable units formed as ISO standard freight containers. Specialized end units provided a rake surface for facilitating movement of the assembly through water. U.S. Pat. No. 5,203,271 to Chapman (1993) disclosed a shallow draft barge for operation in shallow water. U.S. Pat. No. 3,691,974 to Seiford et al. (1972) disclosed a portable barge system having modular pontoon units assembled with a locking system, and U.S. Pat. No. 3,983,830 to Morgan (1975) disclosed a modular barge having tensioned cables for assembling and securing individual barge units.
Other systems have been developed to provide rapid response vessels capable of immediate, emergency deployment. In U.S. Pat. No. 5,479,869 to Coudon et al. (1991), two oil spill recovery barges were each constructed with two pontoons assembled side-to-side. One barge carried a detachable propulsion thrust unit and a detachable crane, and the other towed barge provided storage capacity for collecting recovered hydrocarbons. Although each pontoon was dimensioned for overland truck transport, the assembled barge provided limited functional capabilities for removing oil from the water.
Existing seismic vessels represent significant vessels having large towage and equipment support capabilities, and are not deployable in many regions and water depths of seismic exploration interest. Towed barges do not provide the flexibility to support the multiple functions performed in large marine seismic surveys. There is, accordingly, a need for a seismic vessel capable of deployment in remote and otherwise inaccessible regions. The vessel should be easy to transport but be sufficiently large to support conventional marine seismic equipment.